With a lineup that included modeling, weldability, corrosive wear behavior, filter technology, plasma ladle furnaces and foundry automation, the central element in the steel sessions was how to make better castings while reducing costs.

In "Directions in the Production of Clean Steels," A. Cramb, Carnegie Mellon University, reviewed how to manufacture clean steels with low inclusion content and discussed current steelmaking practices that enable this production.

The presence of significant quantities of inclusions leads to cosmetic surface defects in coated products for exposed application, and to ruptures and porosity during deep drawing for can stock and wire manufacture.

In the most critical applications, the presence of even one inclusion with a diameter of 50 microns and above can lead to a product defect and a rejection by the end user. Due to the need for greater product consistency and reduced external rejects, a number of integrated and electric furnace based steel producers developed techniques and practices to allow steel production of low inclusions and residual element content. Carbon, oxygen, nitrogen, sulphur and hydrogen contents can be closely controlled and reduced to levels measured in parts per million. Inclusion size distributions can be reduced during processing and steels with very low frequencies of inclusions greater than 20 microns are routinely produced.

Cramb said current clean steelmaking and casting practices are based on three principles:

* Oxygen, which is dissolved in liquid steel at the steelmaking and melting stage, must be transformed into a solid or a gas and removed before casting.

* The external oxygen sources that are responsible for the reoxidation of liquid steel must be eliminated at every step in the process.

* The physical entrapment of the liquid fluxes used during steel refining and casting must be eliminated.

He noted that to produce a clean steel, there must be a generation of inclusional material, transport of this material to an interface, and its separation and removal from the interface.

"The production of clean steels depends on an intimate knowledge of the thermodynamics of inclusion formation and the fluid dynamics of inclusion removal," Cramb said. "Transport of inclusions to an interface and separation of inclusions from the liquid steel at that interface determines the effectiveness of the clean steel process."

"In steel foundries, a large proportion of the total production costs are incurred in the melting shop and cleaning room--comprising 40-60% of total operating costs," the authors said. "Each percent increase in yield is a major step toward reaching better profitability."

With requirements of higher quality and closer tolerances, they said, it's becoming difficult to achieve required steel casting qualities by trial and error. This is especially true in designing the gating and feeding systems.

The authors said current runner systems are troublesome because gating design calculations make intensive use of Bernoulli's equation and the law of continuity. Although this method provides a first estimate of mold filling time, it is limited because it doesn't consider metallurgical requirements; assumes steady-state conditions that don't occur during mold filling; and doesn't consider the whole pouring and gating system.

Material and requirement-oriented design of the processes and application to real components can now be realized by computer-aided simulation techniques. A simulation of filling, solidification processes and shrinkage behavior takes into account casting times and material properties and allows for: gating system design for best practice possible in delivering metal to the mold; prediction of potential weak spots in the solidified casting; verification of production tolerance limits; and prediction of properties.

After studying simulation on a 3100 kg carbon steel casting in comparison with conventional riser design, the authors found increased productivity by reducing the amount of metal melted, increased yield, and the elimination of time-consuming and cost-intensive shop trials. Simulation leads to a decline of defects with significant reductions of waste scrap and rework.

They also noted that freezing pattern prediction has some of the most immediate economic returns, and said it's worthwhile applying computation methods even to small-scale production items.

"Improved yield is reflected in a substantial decrease in manufacturing expenses and in increased profitability," the authors concluded.

COPYRIGHT 1993 American Foundry Society, Inc.
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